Double-stranded RNA (dsRNA) molecules are important sequence-specific posttranscriptional regulators of gene expression. dsRNA also plays a role in epigenetic modification of DNA and chromatin. RNA silencing is initiated by dsRNA precursors that are processed to short dsRNA molecules of distinct length and structure. These small dsRNAs associate with cellular proteins that mediate target mRNA or target DNA sequence recognition. Two classes of such RNA molecules have been identified, small interfering RNAs (siRNAs) and microRNAs (miRNAs). While siRNAs are generally complementary to mRNAs and mediate mRNA degradation and chromatin modification, most miRNAs are only partially complementary and are believed to act predominantly as translational regulators. miRNAs represent an extensive class of evolutionary conserved noncoding RNAs of about 22 nucleotides in length that are thought to regulate gene expression in metazoans. The first miRNAs to be identified were the products of the lin-4 and let-7 genes of C. elegans. The miRNA and RNAi pathways are fundamentally related as members of the Dicer and the Argonaute protein families are involved in both of these RNA-mediated silencing processes. siRNAs and microRNA-like siRNA precursors have become important tools for studying gene function in mammalian cells and organisms. The aim of this meeting is to report on progress in understanding the molecular mechanisms underlying siRNA/miRNA function, as well as the technological adaptation of these cellular mechanisms to genome-wide analysis of gene function in mammalian systems and as therapeutic agents.